Direct binding of Talin to Rap1 is required for cell–ECM adhesion in Drosophila

Abstract

Attachment of cells to the extracellular matrix (ECM) via integrins is essential for animal development and tissue maintenance. The cytoplasmic protein Talin (encoded by rhea in flies) is necessary for linking integrins to the cytoskeleton, and its recruitment is a key step in the assembly of the adhesion complex. However, the mechanisms that regulate Talin recruitment to sites of adhesion in vivo are still not well understood. Here, we show that Talin recruitment to, and maintenance at, sites of integrin-mediated adhesion requires a direct interaction between Talin and the GTPase Rap1. A mutation that blocks the direct binding of Talin to Rap1 abolished Talin recruitment to sites of adhesion and the resulting phenotype phenocopies that seen with null alleles of Talin. Moreover, we show that Rap1 activity modulates Talin recruitment to sites of adhesion via its direct binding to Talin. These results identify the direct Talin-Rap1 interaction as a key in vivo mechanism for controlling integrin-mediated cell-ECM adhesion.

Rap1 and membrane lipids cooperatively recruit talin to trigger integrin activation

“…We recently identified a direct 'Rap1-talin' membrane targeting pathway to regulate integrin activity in mammals, which was previously observed in Dictyostelium discoideum (Plak et al, 2016;Zhu et al, 2017). The physiological relevance of this pathway was subsequently demonstrated in mice and flies by us and others (Bromberger et al, 2018;Camp et al, 2018). However, while mutant flies that are deficient in Rab1 binding show embryonic lethality, a phenotype resembling that of talin-null mutants (Brown et al, 2002), Rap1binding mutant mice comprising a talin knockin showed rather mild integrin defects in leukocytes and platelets (Bromberger et al, 2018).…”

Cell Adhesion by Integrins

“…Thus, it appears that pathways upstream of talin (and kindlin) are able to target and activate specific integrin subunits, enabling cells to react differentially to separate outside-in signals. One of these pathways may involve a direct activation of the talin-head domain by Rap1-binding, instead of an indirect, RIAM-dependent mechanisms (58,68,502).…”

Slik phosphorylation of talin T152 is crucial for proper talin recruitment and maintenance of muscle attachment in Drosophila

“…Talin is typically a dimer, and exists in a globular, inactive conformation, or an extended, active conformation (Goult et al, 2013). Both Rap1 and lipid binding play a role in talin activation (Camp et al, 2018;Ellis et al, 2013;Goult et al, 2018;Klapholz and Brown, 2017). In a classical model, active, extended talin binds with its head domain to integrin, and with its rod domain to the actin cytoskeleton (Kanchanawong et al, 2010).…”

“…Both structurally and functionally, Drosophila Talin is highly conserved with vertebrate talins (Ellis et al, 2013). The Talin head plays a particularly important role in adhesion complex stabilization and integrin clustering (Camp et al, 2018;Ellis et al, 2014). In addition, a mutagenesis study revealed functions for additional integrin-binding sites and orientations of Talin with respect to the membrane; however, at muscle attachment sites, Talin orientation is canonical (Klapholz et al, 2015).…”